Ceramic honeycomb structures are widely used as anti-pollutant devices in the exhaust systems of automotive vehicles, both as catalytic converter substrates in automobiles, and diesel particulate filters in diesel-powered vehicles. In both applications, the ceramic honeycomb structures are formed from a matrix of relatively thin ceramic webs which define a plurality of parallel, gas conducting channels. In honeycomb structures used as ceramic catalytic substrates, the cell density may be as high as about 900 cells per square inch. To reduce the pressure drop that the exhaust gases create when flowing through the honeycomb structure, the web walls are rendered quite thin, i.e. on the order 2-6 mils. Ceramic honeycomb structures used as diesel particulate filters generally have a lower cell density of between about 100 and 400 cells per square inch, and are formed from webs on the order of 12-25 mils thick. In both cases, the matrix of cells is preferably surrounded by an outer skin.
Such ceramic honeycomb structures may be formed by an extrusion technique in which an extruded body is cut into segments that form green ceramic bodies. After drying, these honeycomb green bodies are fired at temperatures of at least 1100° C. or higher, and typically 1300° C. or higher in order to sinter the batch constituent particles present in the extruded material into a finished ceramic honeycomb structure. The finished fired honeycomb bodies may be subjected to additional heating steps in which they are fired again to a lower temperatures, for example, on the order of 800° C. or more. The finished ceramic structures may also be subjected to a coating process that coats the gas contacting surfaces with a washcoat, possibly containing catalytic metals. In this application, the term “unfinished” ceramic structure refers to any precursor to a finished ceramic structure, including a dried green body or an unfired or partially fired green body.
Unfortunately, due to the thinness of the outer skin and the inner cell-forming webs, the substantial thermal stresses that the unfinished ceramic structures undergo during the firing processes, and the necessary mechanical handling of the green and fired bodies during the manufacturing process, defects such as internal cracks and voids may occur, as well as separations between the outer skin and the inner matrix of webs. Additionally, upsets due to raw material deviations from specifications may also occur possibly leading to property variations. To reduce the occurrence of such defects, it would be desirable to have a quality control procedure which allowed the manufacturer to reliably trace any defective ceramic honeycomb structure back to the specific factory, kiln, and batch that it originated from and to other processing steps undertaken. Such a procedure would allow the manufacturer to review the particular manufacturing parameters used to fabricate the defective unit and to modify its manufacturing operation in order to reduce the occurrence of such defects in future articles. Accordingly, it is a known procedure to mark, after the final firing or heating step, finished ceramic honeycomb structures with marks containing manufacturing information so that remedial manufacturing operations may be implemented.
Unfortunately, the applicants have observed that such a marking procedure does not reliably result in an accurate recovery of the manufacturing information associated with a particular ceramic honeycomb structure. In particular, the applicants have observed that subsequent to the manufacture of the green bodies of such structures, different batches of green bodies from different kilns may become mixed together in order to efficiently implement other stages of the fabrication process. Hence a quality control process where manufacturing information is printed on the finished ceramic honeycomb structures may not accurately reflect the actual manufacturing conditions and history of the structures, i.e., reliable traceability is not achievable.
To avoid the aforementioned problems, it is necessary to print a data carrying mark on the skin of the green bodies that ultimately form finished completed ceramic honeycomb structures. However, there are a number of problems associated with implementing such a method due to both the fragility of the green bodies, the high temperatures they are subjected to during the firing process, the speed with which they must be marked in order to avoid a production bottleneck, and the tendency of some inks to run or blur when printed on the green body, or to degrade or react with the unfired material forming the skin of the green body.
Accordingly, there is a need for a system and method for printing a data-carrying mark on the skin of a green ceramic honeycomb structure which does not apply potentially damaging pressure on the thin sidewalls of such structures, and which is capable of withstanding the firing temperatures at or above 800° C., at or above 1100° C., or even at or above 1300° C. Ideally, such a method would be capable of printing a unique mark on each one of a particular batch of green ceramic structures, so that the manufacturing history of each particular ceramic honeycomb structure (such as date of manufacture, specific factory, kiln and batch) can be accurately traced. It would be desirable if the information contained in the resulting mark would be maintained even if a portion of the mark were obliterated during the use of the ceramic honeycomb structure.
Such a marking system and method should be rapid and reliable and compatible with high-speed manufacturing techniques so as not to create an expensive production bottleneck. The ink used to form the mark should be nontoxic, and able to survive firing temperatures of at least 800° C., or even 1100° C. or more, or even 1300° C. or more, and be chemically compatible with the unfired ceramic material forming the body. The ink should not blur or run when printed, and it should have similar thermal expansion and contraction properties so as to create a clear mark that does not crack or peel during the firing and cooling steps of manufacture, and does not create excessive thermal stresses. Finally, the ink should not degrade or react with the ceramic material forming the wall of the structure during any phase of the manufacturing process, and should visibly contrast not only against the fired ceramic material forming the finished structure, but also against any catalytic washcoat applied to the structure.